Supercalendered (SC) papers generally are used in a variety of inexpensive color printed media, most often as advertising inserts to newspapers. Calendering is the process by which paper is pressed between two cylinders to impart a smooth surface. The term SC refers to pressures that are sufficiently high so as to impart not only a smooth but also glossy finish to the paper, rendering it suitable for such printing applications.
Compared to most grades of paper, supercalendered papers generally contain high percentages of mineral fillers, as much as 40 weight percent or higher. This is typically done for reasons of economics, as well as to provide a surface that is optimal for printing. Hydrous kaolin clays are comprised of clustered plate-like structures that, after exposure to the calendering tend to break apart into more discrete structures that provide a good printing substrate.
Precipitated calcium carbonate (FCC) is a synthetic pigment, well-known and used by the paper industry, that typically exhibits spheroidal-type, non-platy structures. PCC is often used in filled papers due the high whiteness and brightness it imparts. However, because of its spheroidal shape, PCC often increases the porosity of the sheet. Porous papers typically do not print as well as non-porous ones, and this has been associated with absorption by the porous papers of the liquid vehicles that are employed to transport ink to the paper surface. However, such porosity in paper aids in scattering incident light, thus contributing to desirable properties such as opacity and brightness.
The preferred printing process for SC papers is offset lithography. The basic printing unit of an offset press contains three or sometimes more cylinders: a plate cylinder, to which the plate is attached, a blanket cylinder, to which the offset blanket is attached, and an impression cylinder, which carries the paper through the printing unit and provides a hard backing against which the blanket can impress an image on the paper. Many presses also contain some number of transfer cylinders, which carry the paper either to additional printing units such as in multi-color presses or to the delivery tray.
The arrangement of the cylinders can vary depending on the press, so long as the proper cylinders are adjacent to each other. The plate cylinder is typically the topmost cylinder, and is in contact with the inking and dampening form rollers. The plate contains a right-reading image that is inked by the inking system. As the plate cylinder rotates, it is brought into contact with the blanket cylinder beneath it. The blanket cylinder can be backed away from the plate cylinder, to facilitate removal and adjustment of the plate. The inked image areas of the plate transfer the image to the rubber blanket, the image now being wrong-reading. The paper or other substrate is carried by the feeder system to the impression cylinder, and is sent through the nip between the blanket and impression cylinders. There, the blanket transfers the image to the substrate. The substrate then is transferred to the delivery cylinder if the press is a single-color press or to a series of transfer cylinders which send it to other printing units if the press is a multi-color press.
Among the most common issues that can be encountered in an offset press room printing filled SC papers are piling and picking, and PCC-filled papers tend to be more prone to these than clay-filled sheets.
Piling is a printing problem characterized by the accumulation of paper fibers called lint, bits of detached coating particles, or other debris such as pickouts on the printing plate or blanket, in either image or non-image areas, that affect print quality. Piling can result from a variety of causes in addition to basic surface debris, such as the use of tackier inks that overcome the paper's pick resistance, changes in the blanket's surface stickiness or increased blanket slippage, the chemical composition of the press's dampening system reacting with that of the paper, and various stresses imposed on a paper during feeding. Piling can be classified in a variety of ways, depending on the characteristics of the material. Piling not only affects print quality but can increase the rate of plate or blanket wear.
The term piling also refers to the accumulation on the plate or blanket of dry particles of ink. Ink piling commonly is caused by the inadequate viscosity of the ink vehicle, or an inadequate amount of vehicle, preventing the pigment from being transferred to the substrate with the vehicle and remaining behind on the plate or blanket. In some cases, an overly absorbent substrate will drain the vehicle away from the pigment while on the press, leaving the pigment to pile on the plate or blanket. In cases where the paper is to blame, the only solution is to use different paper. If the ink vehicle is inadequate, the addition of body gum may alleviate the problem, if there is no time for ink reformulation. Ink piling is also called caking.
Piling also refers to dried particles of ink that can collect on the impression roller or other rollers used in gravure printing.
Picking refers to rupturing or other deformation of a paper's surface caused during ink transfer by the force of a sticky ink either separating the layers of a paper, forming a blister-like protrusion in the paper, or removing portions of the paper's coating. Picking occurs when the force of an ink film exceeds the paper's pick resistance, or surface strength. There are two forms of picking: dry picking happens when water is not present; wet picking is the result of decreased pick resistance stemming from exposure to moisture prior to printing. Tearing is an extreme form of picking that rips off the paper surface, leaving a delaminated portion on the press sheet, and splitting is the tearing off of large areas of the paper surface, which then stick to the blanket. Small particles that are picked out of the paper surface are called pickouts. Picking is also called plucking.
Picking also describes a similar problem of gravure printing in which bits of the substrate are transferred to the impression roller or other roller.
The term picking also refers to a printing problem occurring in multi-color flexographic printing in which the plates of successive colors remove bits of the first printed color, commonly caused by printing on still-wet ink. Flexographic picking can be alleviated by ensuring that the first down color has the most rapid drying time.